1. Field of the Invention
This invention relates to a real-image type viewfinder, used in a photographing camera, a video camera and so on, and, in particular, to a real-image type viewfinder for a camera, such as a lens-shutter type camera, which has a viewfinder optical system independent of a photographing optical system.
2. Description of the Related Art
In general, a viewfinder optical system for a real-image type viewfinder includes: a positive objective optical system for obtaining an inverted finder-image; an erecting optical system for converting the inverted finder-image into an erected finder-image; and an ocular optical system for viewing the erected finder-image therethrough.
As is well known, it is necessary to display various information patterns, such as a viewing-field-frame for defining a frame area to be photographed, a photometry-sensing-frame for defining a photometric area to be measured, suitable information symbols and so on, on a viewing field of the viewfinder. Namely, the various information patterns must be superimposed on the finder-image, observed through the ocular optical system of the viewfinder optical system.
Conventionally, in order to superimpose the information patterns on the finder image, a transparent glass plate, on which the information patterns are printed as opaque areas, is placed in the vicinity of a image-forming plane of the positive objective lens system of the viewfinder optical system. Thus, when viewing through the ocular optical system, the finder-image, together with information patterns superimposed thereon, can be observed, due to the transparent glass plate with the information patterns being located at the image-forming plane.
The locating of the transparent glass plate in the viewfinder optical system is relatively easy and inexpensive. Nevertheless, the method utilized is unadvisable, as dust, dirt and other types of particulate matter may be easily deposited on the transparent glass plate. Of course, the adhesion of dust and dirt to the transparent glass plate impairs the visual clarity.
Unexamined Japanese Patent Publication No. 1-113736 discloses another approach for superimposing the information patterns on the finder-image. According to this approach, an optical information-pattern-projecting system is incorporated into the viewfinder optical system, and includes a half mirror interposed between the erecting optical system and the ocular optical system, and a translucent plate, such as a frosted glass plate, having the information patterns printed as transparent-area segments thereon.
The translucent plate is positioned at a location that is the optical equivalent of the location of the image-forming plane of the positive objective optical system with respect to the ocular optical system. The translucent plate is illuminated with natural or external light, in such a manner that light rays, passing through the translucent plate, are directed to the ocular optical system through the half mirror. Thus, when viewing through the ocular optical system, the finder-image, obtained through the objective optical system, can be observed, together with the information patterns, superimposed thereon, which are obtained through the translucent plate.
As mentioned above, the information patterns are printed as transparent-area segments on the translucent plate. Therefore, providing particulate matter is only adhered to the translucent area of the translucent plate, the visual clarity of the ocular optical system is not impaired.
Nevertheless, the approach, disclosed in the above-mentioned publication, has some disadvantages as follows:
Firstly, a focal length of the ocular optical system is increased, due to the existence of the half mirror interposed between the erecting optical system and the ocular optical system, resulting in lowering a magnification of the viewfinder optical system. Thus, in order to compensate for the lowered magnification, a size of the viewfinder optical system must be increased.
Secondly, when the concept of the previously-mentioned publication is applied to an existing camera, a basic design for a viewfinder optical system of the existing camera must be modified, due to the fact that the half mirror must be interposed between the erecting optical system and the ocular optical system. Of course, a modification to the basic design of the viewfinder optical system is very costly.